fix: nil check before GetToken() dereference in MappingNode traversal

parser.go:469 dereferenced GetToken() without checking for nil, causing
a potential panic when traversing MappingNode keys with missing tokens.

Other call sites in the same function already guard with nil checks
(lines 481, 509); this brings the pattern in line with them.

Author: Claude

parser.go

@@ -466,7 +466,11 @@ func findKeyPos(node ast.Node, parts []string) (int, int) {
 	switch n := node.(type) {
 	case *ast.MappingNode:
 		for _, mv := range n.Values {
-			keyVal := mv.Key.GetToken().Value
+			tok := mv.Key.GetToken()
+			if tok == nil {
+				continue
+			}
+			keyVal := tok.Value
 
 			if keyVal != basePart {
 				continue